The field of the invention is that of meters for measuring liquid flow. More specifically, the field is that of turbine meters for fuel dispensers.
Various devices are known in the art for measuring liquid flow. One type of meter which is commonly used in fuel dispensers is a piston type of positive displacement meter. Piston meters require a chamber to accept and then exhaust a liquid. Often the chamber size needed to accommodate the piston meter is a limiting factor for liquid dispensing device, particularly in fuel dispensing installations.
A turbine or rotor meter is another type of liquid flow meter. The turbine meter has, in effect, a small fan or rotor placed within a liquid line. The liquid flow then drives the fan with the fluid flow pressure turning the fan. The rotational velocity of the fan is roughly proportional to the rate at which liquid flows through the meter. The rotational velocity is converted into a train of pulses by a transducer. The pulse rate if therefore roughly proportional to the flow rate.
Turbine meters, due to their small size, are well suited for fuel dispensing devices. However, problems exist with the use of turbine meters. Turbine meters exhibit non-linear behavior, because the observed rotational velocity of the fan is not exactly proportional to the amount of liquid passing through the meter for all rates of fluid flow. At slow and fast flow rates, a turbine meter is therefore often inaccurate. Fuel dispensing devices have little tolerance for erroneous measures; thus to utilize a smaller turbine meter, its measuring error must be corrected.
An example of a prior art instrument for error compensation in a flow meter is found in U.S. Pat. No. 3,965,341 (Honey). Honey describes a flow rate computer which corrects the measured flow rates for liquids having a wide range of viscosities. The Honey flow rate computer uses pre-programmed values taken from a "universal flow meter curve" to correct observed flow rates for various viscosities. It uses a curve which plots correction values against frequency divided by viscosity.
Another instrument is described in U.S. Pat. No. 4,306,457 (Fukui). The Fukui device uses a predetermined table matching rotations per second with an error offset value. The circuit described in the Fukui patent measures the number of rotations which occur during a predetermined interval. It does not estimate the error reduction factor for any point between the number of rotations listed on its table (e.g. 2, 2.5, 3, etc.).
A third instrument for error compensation for a flow meter is described in U.S. Pat. No. 4,581,946 (Kanayama). The Kanayama device performs a form of linear interpolation derived from a predefined flow rate curve. The flow rate curve plots an error coefficient against flow rate. A linear approximation of the flow rate curve is generated by a sum of the least squares analysis. In this manner, the Kanayama device estimates the actual flow rate on the basis of the measured flow rate from the flow meter. However, the Kanayama device uses only one of a predetermined set of predefined compensation constants to correct the measured flow rate.
What is therefore needed is a simple, yet accurate, linearization apparatus which can be used with a turbine flow meter. Also what is needed is a turbine flow meter which reliably determines the flow rate from a set of output pulses which defines the frequency. An additional need is for a flow meter which is adaptable to changing conditions and to a variety of dispensers.
The prior art lacks a small rotory meter which gives accurate results. Also lacking is a method for correcting non-linear output which both accurately measures flow rates and robustly adapts to changing conditions.